1. Field of the Invention
This invention relates to dissipative hermetically sealed electrical filter assemblies which incorporate electromagnetically lossy ceramic materials to provide a low-pass frequency response.
2. Description of the Prior Art
Radio frequency interference (RFI) suppression filters having a low-pass characteristic are commonly incorporated in electrical interconnection devices or in electrical devices as integral subassemblies to insure that unwanted radio frequency signals are suppressed while allowing the passage of direct current (DC) and low frequency alternating current (AC) signals. This RFI suppression function is sometimes required to insure the unimpeded operation of RF sensitive electronic equipment in an intensive RF signal environment or, alternatively, to prevent the conductive or radiative emission of RF energy from electronic devices. The RFI suppression function is of considerable concern in the design of electroexplosive devices (EEDs) where the failure to suppress RF energy might lead directly to the unpropitious functioning of an explosive or propellant charge. Such filters must pass direct currents with negligible internal loss.
In many cases, electrical devices incorporating these RFI filters are also required to provide a gas-tight seal to protect sensitive components or materials contained within an enclosure.
Heretofore, the electrical low-pass filters and the mechanical gas- or liquid-tight seals required by these devices have been separate and distinct components. Many EEDs incorporate a hermetically sealed chamber for their energetic chemical material that is vulnerable to degradation by the intrusion of water vapor. Electrical access to this chamber is obtained by a high integrity glass-to-metal seal that incorporates imbedded electrical thru-conductors, hereafter called electrodes. Similarly, many bulkhead mounted connectors also incorporating RFI suppression filters that are used in aerospace applications are constructed using glass- or ceramic-to-metal sealing techniques to achieve required gas- and liquid-tightness.
Absorptive filters are those that dissipate applied RF power within a solid medium in the form of heat which must be efficiently conducted to the environment. The loss mechanism may be electrical, magnetic or a combination thereof. These lumped- or distributed-element dielectromagnetic structures may be complemented with associated reactive structures (series inductances and shunt capacitances) to achieve desired electrical network characteristics.
Electrically dissipative ceramics formed primarily from alumina and silicon carbide are described in L. E. Gates, Jr., et al. U.S. Pat. No. #3,538,205 issued on Nov. 3, 1970 for "Method of Providing Improved Lossy Dielectric Structure For Dissipating Electrical Microwave Energy," and in L. E. Gates, Jr., et al. U.S. Pat. No. 3,671,275 issued on Jun. 20, 1970 for "Lossy Dielectric Structure For Dissipating Electrical Microwave Energy." Electrical loss tangents as high as 0.6 are reported. L. E. Gates, Jr., et al. U.S. Pat. No. 3,765,912 issued on Oct. 16, 1973 for "MgO-SiC Lossy Dielectric for High Power Electrical Microwave Energy" reports a further development based on a matrix of magnesia and silicon carbide. However, these compositions feature negligible magnetic loss, high porosity, high melting points, and poor wetting characteristics when in the liquid state. As such, they are unsuitable for forming fusion seals with metallic members.
Magnetically dissipative materials having acceptably high magnetic loss tangents and DC volume resistivities are commercially available in the form of spinel ferrites. E. C. Snelling in Soft Ferrites. Properties and Applications (Second edition) (Butterworths, Stronham Mass., 1988) describes the electromagnetic properties of these materials. P. Schiffres in "A Dissipative Coaxial RFI Filter", IEEE Transactions on Electromagnetic Compatibility (January 1964, pp. 55-61), describes the application of these materials for constructing lossy transmission line filters and J. H. Francis, in "Ferrites as Dissipative RF Attenuators," Technical Memorandum W-11/66, U.S. Naval Weapons Laboratory, Dahlgren Va. (1966), describes their application as EED attenuation elements.
Various glass sealing compositions have been developed for bonding ferrite shapes to one another as reported in J. F. Ruszczyk U.S. Pat. No. 3,681,044 issued on Aug. 1, 1972 for "Method of Manufacturing Ferrite Recording Heads With a Multipurpose Devitrifiable Glass," R. Huntt U.S. Pat. No. 4,048,714 issued on Sep. 20, 1977 for "Glass Bonding or Manganese-Zinc Ferrite," and Y. Mizuno et al. U.S. Pat. No. 4,855,261 issued on Aug. 8, 1989 for "Sealing Glass." These compositions do not feature the electromagnetically lossy characteristics that would render them useful as RF absorbers.
J. A. Pask discusses CHEMICAL BONDING AT GLASS-TO-METAL INTERFACES in an article published in the TECHNOLOGY OF GLASS, CERAMIC, OR GLASS-CERAMIC TO METAL SEALING presented at The Winter Annual Meeting of the American Society of Mechanical Engineers, Boston, Mass., Dec. 13-18, 1987. This paper discloses that the fusion joint interface between a reflowed glass-like ceramic and the substrate to which it is bonded, be it a ferrite or a metal structure, is a chemically distinct region.
Assemblies incorporating magnetically lossy RF absorptive filter elements, typically spinel ferrites in the form of sintered beads, and physically distinct mechanical seal elements, typically fused glass-to-metal structures, are described in T. Warnhall U.S. Patent No. 3,572,247 issued on Mar. 23, 1971 for "Protective RF Attenuator Plug for Wire-Bridge Detonators," J. A. Barret U.S. Pat. No. 4,422,381 issued on Dec. 27, 1983 for "Ignitor With Static Discharge Element and Ferrite Sleeve," and H. W. Fogle U.S. patent application Ser. No. 07-706211 executed on May 28, 1991, for "Filtered Electrical Connection Assembly Using Potted Ferrite Element." These designs require separate processing steps to form the filter and seal elements.
Assemblies incorporating electrically lossy RF absorptive filter elements, typically ferroelectric materials such as Barium Titanate (BaTiO.sub.3) in the form of tubular capacitors, and physically distinct mechanical seal elements are described in W. G. Clark U.S. Pat. No. 3,840,841 issued on Oct. 8, 1974 for "Electrical Connector Having RF Filter," K. S. Boutros U.S. Pat. No. 4,187,481 issued on Feb. 5, 1980 for "EMI Filter Connector Having RF Suppression Characteristics," and S. E. Focht U.S. Pat. No. 4,734,663 issued on Mar. 29, 1988 for "Sealed Filter Members and Process For Making Same."
Certain automotive spark plugs unify the RF filter and mechanical seal functions in a glassy ceramic structure that forms a fused seal. For example, G. L. Stimson U.S. Pat. No. 4,112,330 issued on Sep. 5, 1978 for "Metallized Glass Seal Resistor Compositions and Resistor Spark Plugs," K. Nishio et al. U.S. Pat. No. 4,224,554 issued on Sep. 23, 1980 for "Spark Plug Having a Low Noise Level," M. Sakai U.S. Pat. No. 4,504,411 issued on Mar. 12, 1985 for "Resistor Composition For Resistor-Incorporated Spark Plugs," and G. L. Stimson U.S. Pat. No. 4,795,944 issued on Jan. 3, 1989 for "Metallized Glass Seal Resistor Composition," describe ceramic composition hermetic seals that also act as series connected electrically dissipative resistances, typically 5000 ohms, to attenuate RF energy generated at the spark gap so as to reduce RFI emissions from the vehicle ignition system. These designs depend entirely upon ohmic and dielectric loss mechanisms to dissipate RF energy. More significantly, they do not have metallic electrically conducting electrodes that pass through the glassy seal region with the result that DC losses are significant. These factors render this technology useless for the manufacture of electrical thru-bulkhead fittings, connectors and EEDs where DC continuity is an essential performance requirement.
Plastics with ferrimagnetic or ferroelectric fillers that are intended for use as RF signal attenuating media are described in H. J. Sterzel U.S. Pat. No. 4,879,065 issued on Nov. 7, 1989 for "Processes of Making Plastics Which Absorb Electromagnetic Radiation and Contain Ferroelectric and/or Piezoelectric Substances." Such plastics allow the design of attenuating filters that have imbedded electrodes shaped in useful inductive configurations, e.g. spirals and helical windings. However, these materials do not have the mechanical durability and chemical resistance required for mechanical gas- and liquid-tight seals, particularly at extreme hot and cold temperatures or in corrosive environments.
Filters featuring spiral shaped electrodes imbedded in lossy ferrimagnetic ceramics are reported in Dow et. al. U. S. Pat. No. 4,848,233 issued on Jul. 16, 1989 for "Means For Protecting Electroexplosive Devices Which Are Subject To A Wide Variety Of Radio Frequency." These fragile high-porosity devices can not simultaneously serve as fluid sealing elements.
While filter/seal equipped thru-bulkhead fittings, connectors, EEDs and spark plugs such as those described in the prior art patents have met with considerable success, they nevertheless suffer from the disadvantage of complexity in that they require a multiplicity of constituent parts and various means for joining same together to achieve the electrical, mechanical and heat transfer functions intended. This complexity leads to significant manufacturing cost, particularly if the filter designs are not amenable to assembly by high speed machinery.